Mutations in mitochondrial ferredoxin FDX2 suppress frataxin deficiency.

Abstract

Frataxin is a key component of an ancient, mitochondrial iron-sulfur cluster biosynthetic machinery, serving as an allosteric activator of the cysteine desulfurase NFS1 (refs. ^1-5). Loss of frataxin levels underlies Friedreich's ataxia⁶, the most common inherited ataxia. Yeast, Caenorhabditis elegans and human cells can tolerate loss of frataxin when grown in 'permissive' low oxygen tensions⁷. Here we conducted an unbiased, genome-scale forward genetic screen in C. elegans leveraging permissive and non-permissive oxygen tensions to discover suppressor mutations that bypass the need for frataxin. All mutations act dominantly and are in the ferredoxin FDX2/fdx-2 or in the cysteine desulfurase NFS1/nfs-1 genes, resulting in amino-acid substitutions at the FDX2-NFS1 binding interface. Our genetic and biochemical analyses show that the suppressor mutations boost iron-sulfur cluster levels in the absence of frataxin. We also demonstrate that an excess of FDX2 inhibits frataxin-stimulated NFS1 activity in vitro and blocks the synthesis of iron-sulfur clusters in mammalian cell culture. These findings are consistent with structural and biochemical evidence that frataxin and FDX2 compete for occupancy at the same site on NFS1 (refs. ^8,9). We show that lowering levels of wild-type FDX2 through loss of one gene copy can ameliorate the growth of frataxin mutant C. elegans or the ataxia phenotype of a mouse model of Friedreich's ataxia under normoxic conditions. These genetic and biochemical studies indicate that restoring the stoichiometric balance of frataxin and FDX2 through partial knockdown of FDX2 may be a potential therapy for Friedreich's ataxia.